The First Officer became incapacitated during the incident, forcing the Captain to descend further to 8000ft and make an emergency landing in Canberra shortly after midnight.

Both pilots were taken to hospital as a precaution.

No passengers were on board the freight service and there was no damage to the aircraft during the incident or landing.

The VH-XMO cargo plane was part of the Express Freighters Australia fleet which is owned by Qantas and run out of Sydney.

The ATSB will collect and examine information on the aircraft's flight data recorders and interview maintenance and flight crew as part of the investigation into the "serious incident".

"Should a critical safety issue be identified during the course of the investigation, the ATSB will immediately notify those affected and seek safety action to address the issue," the ATSB statement read.

A final report of the investigation is expected by the end of the year.